One method of welding workpieces constituted by two members is a method referred to as friction stir welding. The friction stir welding is a method of generating frictional heat at workpieces surface by rotating a tool at a welding place of the workpieces in a state in which a predetermined welding pressure is applied to a surface, which is referred to as a shoulder surface of the tool, and softening and welding the workpieces by the frictional heat. The friction stir welding may include a method of using a tool, which is referred to as a bobbin tool type.
The bobbin tool-type tool has a surface side shoulder having one shoulder surface, and a rear surface side shoulder having another shoulder surface opposite to the one shoulder surface. The rear surface side shoulder is attached to a shaft section passing through the surface side shoulder. Then, when the friction stir welding is performed, the shaft section passes through the workpieces, the surface side shoulder is disposed at a surface side of the workpieces, and the rear surface side shoulder is disposed at the rear surface side. Then, while the workpieces is softened by pressing the workpieces to sandwich the surface and the rear surfaces of the workpieces between the shoulder surfaces of the surface side shoulder and the rear surface side shoulder to generate frictional heat, the shaft section inserted into the softened area is stirred, performing the friction stir welding.
Here, an error may be generated at a relative position of the workpieces surface with respect to the tool by a board pressure variation due to distortion or a manufacturing error of the workpieces, or a setting error to a surface plate. When the error is generated at the relative position of the workpieces surface with respect to the tool, a welding force applied to the workpieces from the shoulder surface of the tool is varied, and as a result, a welding error may occur. For this reason, in a hydraulic pressure biasing path configured to generate a welding force applied to the workpieces by the tool, a technique in which a feedback circuit configured to control to a certain pressure is provided has been proposed (for example, see Patent Documents 1 and 2).